Document: Really load the table privileges.

	* glom/libglom/db_utils.[h|cc]: add_group(): Take a bool superuser
	parameter.
	add_groups_from_document(): Use add_group() instead of executing
	the ADD GROUP query directly, to make sure that we also start
	with some sane table privileges defaults, in case nothing else
	sets the privielges.
	* glom/libglom/document/document.cc: load_after(): Fix a typo
	so that the group privileges are really loaded (and then
	saved again instead of being lost).
	* examples/example_film_manager.glom:
	* examples/example_smallbusiness.glom: Save these with
	useful group privileges, so the groups will be allowed to
	see tables.
	* tests/test_document_load.cc: Test the loading of the group
	privileges.
	Previously the examples were created with groups that could not
	even view the tables, which meant that they could not even
	find out about their existence.
	This deals with the problem found here:
	https://bugzilla.gnome.org/show_bug.cgi?id=669043#c2
	But I would like to show the existence (and structure) of tables
	even if their data cannot be viewed, so there is more work to do.